ANNTJLOSA : CRUSTACEA. 185 



in the DapJmia pulex (Jig. 51), or ' branched-horned 

 Water-flea,' which occurs commonly in our ponds, the body is 

 enclosed in a bivalve shell, which is not furnished with a 

 hinge posteriorly, and which opens anteriorly for the protrusion 

 of the feet. The head is distinct, not enclosed in the carapace, 

 and carrying a single eye. The mouth is situated on the 

 under surface of the head, and is provided with two mandibles 

 and a pair of maxillae. The gills are in the form of plates, 

 attached to the five pairs of thoracic legs. The males are 

 very few in number, compared with the females, and a single 

 congress is all that is required to fertilise the female for life. 

 Not only in this case, but the young females produced from 

 the original fecundated female appear to be able to bring forth 

 young without having access to a male. In this way the 

 influence of a single fecundation appears to be transmitted 

 through several generations. 



ORDER II. PHYLLOPODA. Crustacea, mostly of small size, 

 the carapace protecting the head and thorax, or the body 

 entirely naked. Feet numerous, never less than eight pairs, 

 mostly foliaceous or leaf- like, branchial in function. The eyes 

 sometimes confluent, sometimes distinct and sub-pedunculate. 



The Phyllop&da are chiefly interesting from their affinity to 

 the extinct Trilobites. In the typical genera Limnadia and 

 Apus, the body is protected by a carapace which is bivalve in 

 the former and shield-like in the latter. In Limnadia the 

 carapace covers the greater part of the body, and opens along 

 the ventral margin. There are from 18 to 30 pairs of mem- 

 branaceous and respiratory feet. In Apus the carapace is* 

 clypeiform and covers a portion of the abdomen, and there 

 are sixty pairs of feet, of which all but the first pair are 

 foliaceous. 



In Branchipus, which occurs in many pools, the body is not 

 protected by a carapace, and this is also the case with the 

 singular little Artemia salina which inhabits the brine-pans 

 at Lymington and elsewhere. 



ORDER III. TRILOBITA. This order is entirely extinct, none 

 of its members having survived the close of the Palaeozoic 

 period. It is probable that the Trilobites should be placed 

 near the Phyllopoda, but their exact position is uncertain, as 

 no traces of any appendages of any kind, except the labruni, 

 have hitherto been discovered in any Tribolite. 



The body of a Trilobite (fig. 52), was covered with a 

 ' crust,' or exoskeleton, which shows more or less markedly a 

 division into three longitudinal lobes, from the presence of 

 which the name of the order is derived. The shell is com- 

 posed of a cephalic shield, a certain numbe* 1 of free and 



